This disclosure is related to methods of directed self-assembly to form layered structures, and more specifically, to the use of photoresists for directed polymer self-assembly.
Patterning features with smaller critical dimensions allows denser circuitry to be created and therefore can reduce overall production cost. Tighter pitch and smaller critical dimensions are needed for each technology node. Directed polymer self-assembly (DSA) is a potential candidate to extend current lithography by enhancing the spatial resolution of a predefined pattern on the substrate. A graphoepitaxial technique, where the polymer self-assembly is guided by lithographically pre-patterned substrates with topographical features and their surface properties, is one method for implementing DSA. One of the major obstacles impeding integration of graphoepitaxy into standard 193 nm optical lithographic processes is the compatibility of the pre-patterned 193 nm positive photoresist substrate with the organic casting solvent used to apply the self-assembly (SA) polymer solution. Graphoepitaxial pre-patterns are typically generated in a hard mask layer (for example, silicon oxide) to prevent the organic solvent used in casting the self-assembly material (SA material) from dissolving the pre-pattern. The use of hard mask pre-patterns adds process complexity, as shown in the series of steps in FIG. 1A to 1D (Prior Art) using schematic layer diagrams. First, a hard mask layer 202 is deposited on a bottom layer 200 (e.g., silicon wafer or transfer layer), followed by a photoresist layer 204 (FIG. 1A). Patterning the photoresist layer in a lithographic process produces a patterned photoresist layer 206 (FIG. 1B). If the hard mask layer 202 does not also serve as an anti-reflective coating (ARC), a separate ARC layer can be applied to the substrate prior to the photoresist. The image of the patterned photoresist layer 206 comprising trench area 208 is then transferred into the hard mask layer 202 by an additive or subtractive process, after which the photoresist is stripped, leaving a hard mask pre-pattern 210 (FIG. 1C) for self-assembly. Hard mask pre-pattern 210 includes all surfaces (e.g., surfaces 203, 205, and 207 in FIG. 1C) formed or uncovered by etching the hard mask layer and removing the photoresist pattern. If desired, the surface of the hard mask pre-pattern 210 can be further modified prior to applying a material capable of self-assembly. For example, a polymer can be chemically grafted onto the surface to provide the appropriate surface properties for the subsequent self-assembly process. A SA material is then cast from an organic solvent onto the hard mask pre-pattern 210 to form layer 212, in this case allocating the SA material substantially in trench 208. The hard mask pre-pattern topographically directs the self-assembly of the SA material, forming ordered domains 214 and 216 in the trench areas 208 (FIG. 1D). Finally, the hard mask layer must be removed, typically adding another process step. The resulting structure is also difficult to rework. The additional steps needed to generate a hard mask pattern and, optionally, modify its surface properties increases the cost of the DSA process, and further increases process complexity.
Alternatively, a crosslinking negative-tone photoresist that after exposure does not dissolve in the organic casting solvent for the SA material can be used to guide self-assembly of polymers. Unfortunately, negative-tone photoresists that rely on a cross-linking mechanism often suffer from poor profiles, microbridging, and swelling caused by organic solvents. These crosslinked negative-tone resists are difficult to remove if the wafer needs to be reworked. In addition, for specific patterns such as contact holes and vias, positive-tone imaging is preferred. As a result, positive-tone resists are the mainstream resists for high resolution 193 nm patterning and, therefore, most photomasks are designed for positive-tone imaging.
The pre-patterns formed using standard 193 nm positive-tone resists following exposure and aqueous base development are soluble in many organic solvents. This limits their application in DSA due to their incompatibility with the organic solvents used to apply a material capable of self-assembly; that is, the organic solvents can dissolve the pre-pattern. Conventionally, organic solvent compatibility can be achieved by a “freezing” or “hardening” process that creates a non-soluble layer or crust at the photoresist surface. Typically, this can be achieved by a number of processes including crosslinking by damaging radiation (e.g., 172 nm radiation), reaction with a freezing material, or by a surface curing agent that crosslinks the surface of the photoresist. The frozen or hardened positive photoresist is insoluble in the SA organic solvent, but is more difficult to rework. A freezing material or surface curing agent can react with not only the photoresist material, but also with the non-covered substrate surface (e.g., ARC layer or hard mask). In the case where one desires to use chemical differences between the underlying surface and the photoresist sidewall to direct self-assembly, this non-selective reaction is disadvantageous. The freezed photoresist may reflow when baking at a high temperature which may be required to reduce the DSA process time. In addition, the use of a freezing material or surface curing agent imposes significant additional materials and process costs.
Therefore, methods that will circumvent the organic solvent incompatibility without crosslinking the photoresist is highly desirable for integration of DSA into standard lithography.